Have you checked for driver / BIOS / Windows updates?
Is your computer set to reboot on BSODs? If so, disable that so you can see any error messages, if that is why the computer is rebooting.
In the Control Panel, click on System (or 'System and Security' and then System, in category view) and then 'Advanced system settings' in the left pane. In the Startup and Recovery section, click Settings, then uncheck Automatic restart and click OK.
For the Steam version, did you try verifying local files? In the Steam library, right click on Divinity Original Sin and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
If you create a new profile, start a new game with a character having a flare spell, does that cause a reboot casting it into a poison cloud in the tutorial cave?
In a new or existing save have you tried tossing a lit candle into a poison cloud? What about into a poison cloud where there is an open poison vent (which will fuel repeated explosions, unless that was changed recently). The tutorial cave and the cellar of the King Crab both have such poison vents.
Have you tried digging up the grave by the clinic in Cyseal? (opposite side from the marketplace, in the fenced in area)
